Current emission control regulations necessitate the use of catalysts in the exhaust systems of automotive vehicles in order to convert carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) produced during engine operation into unregulated exhaust gasses. Vehicles equipped with diesel or other lean burn engines offer the benefit of increased fuel economy, however, catalytic reduction of NOx emissions via conventional means in such systems is difficult due to the high content of oxygen in the exhaust gas. In this regard, Selective Catalytic Reduction (SCR) catalysts, in which NOx is continuously removed through active injection of a reductant, into the exhaust gas mixture entering the catalyst, are known to achieve high NOx conversion efficiency. Typically, reductant, such as aqueous urea, is carried on board of a vehicle, and an injection system is used to supply it into the exhaust gas stream entering the SCR catalyst where it decomposes into hydro cyanic acid (HNCO) and gaseous ammonia (NH3). The amount of reductant injection is usually determined based on operating conditions, such as engine speed, load, catalyst temperature, and on the NOx conversion efficiency of the catalyst, which can be monitored by coupling a pair of NOx sensors upstream and downstream of the SCR catalyst.
The inventors herein have recognized that in order to achieve high NOx conversion efficiency in such systems, it is important to accurately and quickly diagnose degradation of the upstream and downstream NOx sensors. The inventors have further recognized that under some operating conditions, such as when the SCR catalyst is very cold or very hot, its NOx conversion efficiency is substantially zero. Therefore, under those circumstances, the NOx sensor readings upstream and downstream of the catalyst are expected to be substantially the same.